By Ed Korczynski
Global industry R&D hub IMEC defines the “IMEC 7nm-Node” (I7N) for finFETs to have 56nm Contacted Gate Pitch (CGP) with 40nm Metal Pitch (MP), and such critical mask layers can be patterned with a single exposure of 0.33 N.A. EUVL as provided by the ASML NXE:3400B tool. To reach IMEC 3nm-Node (I3N) patterning targets of ~40 CGP and ~24 MP, either double exposure of 0.33 N.A. EUVL would be needed or else single-exposure of 0.55 N.A. EUVL as promised by the next-generation ASML tool. All variations of EUVL require novel photoresists and anti-reflective coatings (ARC) to be able to achieve the desired patterning.
The Figure shows that IMEC has led tremendous progress on the photoresists, with best resolution in a single 0.33 N.A. EUVL exposure of 13nm half-pitch (HP) line arrays. The most important parameter for the photoresist is the sensitivity target of 20 mJ/cm2, but at that dosage the best materials seen today have unacceptably high line-width roughness of >5nm three-sigma.
“If you’re talking about lines of 16nm width, for 3-sigma you want to be less than 3nm line-width-roughness,” explained Steegen during the 2017 IMEC Technology Forum. “Smoothing techniques are post-develop technologies that basically reduce line-width-roughness. We are working with many partners, and all are making progress in reducing line-width roughness though post-develop techniques.”
Top-down SEM images of the best achieved EUVL resolutions using 0.33 N.A. stepper and Chemically-Amplified Resist (CAR) or metal-oxide Non-Chemically-Amplified Resist (NCAR) formulations, along with post-development “smoothing” technologies to improve the Line-Width Roughness (LWR) to meet target specifications. (Source: IMEC)
The Figure also shows that IMEC has been working with vacuum deposition companies on atomic-layer deposition (ALD) or chemical-vapor deposition (CVD) processes to ideally take off 2 nm of sidewall roughness. Plasma energy may be capacitively- or inductively-coupled to a vacuum chamber to allow for either PEALD or PECVD processing. Such precise atomic-scale processing may be composed of “dep/etch” sequences of one/few atomic layer depositions followed by light plasma etching such that the nominal line-width would not necessarily change. However, this approach necessitates that the wafer leave the lithography track and move to a separate vacuum-tool.
To save on cost and time, LWR smoothing may be accomplished to some extent today in the litho track by specialized spin-on materials. Companies that supply lithography resolution extension (EXT) materials such as spin-on hard masks (SOHM) and anti-reflective coatings (ARC) have looked at ways spin-on materials can improve the LWR of post-developed resist lines. This can be combined with “shrink” materials that add controlled thicknesses to sidewalls of holes, or with “trim” materials that subtract controlled thicknesses from the sidewalls of lines. Generally, some manner of complex chemical engineering is used to create a film that either forms or breaks bonds when thermally driven by a bake step, and after image transfer to underlying SOHM layers the shrink/trim material is typically stripped in a solvent such as propylene glycol methyl ether acetate (PGMEA).
EUVL photoresists may be based on metal-oxide nano-particles, instead of on extensions to the Chemically-Amplified Resist (CAR) formulations that have been mainstays of ArF/ArFi lithography for decades. Inpria Corp.—the 10-year-old-start-up supported by industry—has ultimately developed a tin-oxide family of blends that are shown as the Non-Chemically-Amplified Resist (NCAR) in the Figure. NCAR metal-oxide resists show similar LWR at similar exposure doses to CARs. However, the metal-oxides in the NCAR can often replace SOHM materials, saving cost and complexity in the resist stack.
IMEC’s work on EUVL with ASML steppers leads to the belief that the source power will increase to allow throughput to rise from today’s ~100 wph to ~120 wph by the end of this year. However, those throughputs assume 20mJ/cm2 resist-speed, and masks may require 30 mJ/cm2 target exposures even with post-develop smoothing steps.
[DISCLOSURE: Ed Korczynski is also Sr. Technology Analyst with TECHCET Group, and author of the “Critical Materials Report: Photoresists and Extensions and Ancillaries 2017”.]